1. Field of the Invention
The present invention relates to a method for preparing a PLZT transparent ceramic which has a composition represented by the following general formula: EQU (Pb.sub.1-x La.sub.x)(Zr.sub.1-y Ti.sub.y).sub.1-0.25x O.sub.3
(wherein x is a numerical value ranging from 0.01 to 0.3 and y is a numerical value ranging from 0.05 to 0.95) as well as the PLZT transparent ceramic per se obtained according to the foregoing method.
The PLZT ceramics have been anticipated as materials which may widely be used in various fields, for instance, optoelectronic materials such as those for use in making a photoshutter, a picture image-accumulating device and a light modulation element; electrostrictive materials and piezo-electric materials.
2. Description of the Prior Art
It has conventionally been known that PLZT transparent ceramics have been produced by preparing a powdery calcined raw material of PLZT according to a dry-grinding method, a coprecipitation method or an alkoxide method; forming the powder into a molded product having a desired shape; and then subjecting the molded product to hot-pressing or to a normal pressure-calcination under an oxygen gas atmosphere.
The dry-grinding method comprises mixing compounds (oxides) each containing Pb, La, Zr or Ti and then calcining the resulting mixture. If the calcined powder is formed into a molded product having a desired shape and then subjected to pressure-calcination called hot pressing, a PLZT ceramic having excellent quality can be obtained (see, for instance, Journal of American Ceramic Society, 1971, 54, No. 1, p. 1). However, the hot-pressing method suffers from problems such that molded articles having any desired large shape cannot be produced and that the mass production thereof is quite difficult since the size of dies is limited to a specific level. For this reason, there has been desired the development of a method for preparing PLZT calcined powder which can be subjected to the normal pressure-calcination in an oxygen gas atmosphere.
To solve the foregoing problems, there have been developed wet methods such as a coprecipitation method and an alkoxide method. For instance, Japanese Patent Unexamined Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 62-158117 discloses a method for preparing PLZT calcined powder capable of being easily sintered which comprises the steps of precipitating Pb, La, Zr and Ti by coming bringing solutions of compounds containing these elements in contact with a precipitate-forming solution and then calcining the resulting precipitates containing the foregoing four elements to give the intended PLZT calcined powder. If the PLZT calcined powder is calcined at ordinary pressure in a desired atmosphere, a PLZT ceramic having a high light transmittance comparable to that of the PLZT ceramics obtained according to the hot-pressing method can be obtained. However, this method comprises complicated processes and the raw material to be used is expensive. Under such circumstances, there has been desired the development of a method for preparing a PLZT ceramic which comprises the steps of preparing PLZT calcined powder from a cheap raw material according to a dry method which is easily practicable, then forming the powder into a molded product having a desired shape and calcining the molded product at ordinary pressure in a desired atmosphere.
The technique disclosed in J.P. KOKAI No. Sho 64-79068 comprises the steps of introducing powder for forming a Pb vapor-containing atmosphere which comprises lead zirconate (PbZrO.sub.3) and lead oxide (PbO) into a reactor of aluminum provided with a cover, placing an MgO plate on the powder for forming a Pb vapor-containing atmosphere, placing a molded product obtained from PLZT powder on the MgO plate to thus subject the molded product to a normal pressure-calcination in an oxygen gas atmosphere. If this method is adopted, there can be obtained a PLZT ceramic having a relatively high light transmittance from PLZT calcined powder prepared by a dry method. However, PLZT powder obtained according to the dry method has a particle size greater than that of the powder obtained according to a wet method and, therefore, the sintering properties thereof are insufficient. For this reason, this method suffers from a problem of low reproducibility and the resulting product does not have sufficient light transmittance.